Ophthalmic lenses may be created using a variety of methods, one of which includes molding. In a double sided molding process, the lenses are manufactured between two molds without subsequent machining of the surfaces or edges. Such mold processes are described, for example in U.S. Pat. No. 6,113,817, which is expressly incorporated by reference as if fully set forth herein. As such, the geometry of the lens is determined by the geometry of the mold. Typical molding systems include cast molding, which involves using two mold halves, and spin-casting. These methods may also be combined with other machining techniques to create specific lens designs. Another process involves cycling lenses through a series of stations on a semi-continuous basis. The cyclic portion of lens production generally involves dispensing a liquid crosslinkable and/or polymerizable material into a female mold half, mating a male mold half to the female mold half, irradiating to crosslink and/or polymerize, separating the mold halves and removing the lens, packaging the lens, cleaning the mold halves and returning the mold halves to the dispensing position. Once a mold is designed and fabricated it must be measured to ensure that it meets the proper specifications. Typical molds may be spherical or non-spherical, depending upon the type of lens to be created. Because most molds have one or more arcuate surfaces, linear coordinates may be unable to measure a curved surface accurately or may only be able to accurately measure portions of the mold geometry. Additionally, current measurement means such as Michelson interferometers may be adapted for use in a lab but may not be practical or efficient for use on a manufacturing line due to vibration and other types of interference/noise that may affect sensitive equipment.
An interferometer is a measurement instrument that utilizes optical interference to determine various characteristics of optical surfaces. Interferometers typically generate a precise monochromatic wavefront, such as that of a laser, and split it using a beam splitter. The resulting two wavefronts include a test wavefront and a reference wavefront. These wavefronts are passed through a sample and a reference optical system, respectively, to create interference fringes which may then be measured. Methods for measuring the thickness of a material using interferometers are known in the prior art. For example, U.S. Pat. No. 3,319,515 (Flournoy) relates to the determination of thickness on the basis of interferometric optical phase discrimination and is expressly incorporated by reference as if fully set forth herein. U.S. Pat. No. 5,473,432 (Sorin) and U.S. Pat. No. 5,610,716 (Sorin et al) relate to an apparatus and method for measuring film thickness of a moving film, employing optical reflectrometry, both of which are expressly incorporated by reference as if fully set forth herein.